Articles such as impeller wheels used in compressors, pumps, gas turbine engines and the like are conventionally manufactured from a monolithic or single piece casting that often requires the pouring of over 45 kilograms (100 pounds) of molten air melt alloy into a ceramic mold to yield a casting typically half the pouring weight. The relative massive hub of such a one piece casting provides a concentrated molten heat source that slows the solidification rate of the casting, often resulting in severe burn-in by the molten metal into the mold face coat due to the presence of oxygen, causing poor surface finish in the flow path of the casting. This is of great concern when the flowpath is configured as a shrouded impeller, where the shroud greatly limits access to the flowpath to blend smooth the affected surface. In addition, when cast as one-piece, the massive central hub acts as a heat source during cooling of the impeller wheel casting, slowing down the solidification rate of the adjacent thinner outer vane impeller shroud. Undesirable, this causes the vanes to have relatively poor dimensional repeatability due to uncontrolled variations in the solidification pattern. This results in a wide variation in air flow characteristics of the impeller wheel which is detrimental to the performance of the compressor, pump, or gas turbine engine.
An impeller wheel assembly comprising a hub and a vane ring having adjacent mating surfaces metallurgically bonded together by hot isostatic pressing overcomes many of the manufacturing problems associated with a one-piece cast impeller wheel. Hot isostatic pressing, or HIP bonding as it is well known in the art, is a process in which all the exposed surfaces of the impeller wheel assembly would be subjected to the direct application of elevated temperature and pressure to press the mating surfaces of the vane ring and hub together to form a diffusion bond therebetween so as to establish a highly efficient metallurgical joint. Typically, the pressure is applied through an inert argon gas in a pressure vessel, e.g., an autoclave.
As is known in the art, in order to obtain a metallurgically sound diffusion bond it is necessary to have the outer perimeter of the mating surfaces to be bonded sealed during hot isostatic pressing in a HIP autoclave, so that only the exposed exterior surfaces of the hub and vane ring will be subject to the autoclave pressure and temperature to effect the desired bond. U.S. Pat. NO. 4,096,615 which issued on Jun. 27, 1978 to Cross and U.S. Pat. No. 4,152,816 which issued on May 8, 1979 to Ewing each recognize the problem of sealing the interface cavity or gap between the hub and the impeller casting prior to HIP bonding. Both Cross and Ewing use a brazing alloy to seal the gap. Ewing also shows a passive method of helium leak testing prior to HIP bonding.
In U.S. Pat. No. 4,581,300 which issued on Apr. 8, 1986 to George S. Hoppin III et al, the interface surfaces between the hub and the blade ring are positively sealed by a deformable plate that is electron-beam welded and subsequently brazed to the blade ring to seal the dissimilar metal hub inside a blade ring cavity. After HIP bonding, the plate is removed by machining, and the wheel finish machined.
One of the major difficulties associated with HIP bonding of cast shapes is the occurrence of through-wall or through-seal discontinuities which allow HIP pressure to leak into the interface cavity intended to be bonded, thereby equalizing the internal and external pressure so as to defeat the HIP bonding process.
The problem is more evident in large ferrous-based castings, which often have through wall discontinuities such as micropores that are not detectable by conventional fluorescent penetrant and radiographic methods. Weld and braze sealing of the joint between the hub and vane ring have similar problems. Even through-wall porosity may not have been detected prior to HIP bonding, the external pressures and temperatures of the HIP cycle sometimes cause membranes of a porosity chain to rupture, thereby allowing external pressure to reach the interface cavity so as to defeat the bonding process.
Typically, after the HIP cycle has been completed, its failure or success is usually determined by machining through the welded or brazed seals to expose the bond interface. If any evidence of the interface between the hub and vane ring can be found, as by conventional non-destructive test methods such as fluorescent penetrant inspection, the bond is considered a failure. The source of such failure is usually only a matter of speculation, and the HIP procedures, including separating the hub and vane ring, re-cleaning, re-assembly and re-sealing, is repeated with what is now a low likelihood of success. Often the vane ring is distorted or otherwise damaged during removal of the hub, resulting in considerable expense of labor, time, equipment, waste of natural 10 resources, and a loss of a major expensive component of the impeller wheel assembly.
Heretofore, the inability to readily verify the vacuum integrity of the pre-HIP impeller wheel assembly, and the inability to readily verify a successful bonding cycle, have prevented HIP bonding of two piece impeller assemblies from achieving the status of a viable production process.
The present invention is directed to overcome one or more of the problems as set forth above.